Continuous casting is a process in which molten metal is continuously poured into a mold, cooled and made to solidify in order to supply a molded piece in a predetermined shape.
Apparatuses for measuring a level of molten metal in the mold using electromagnetic waves have been proposed (Patent Documents 1 to 3). Such conventional electromagnetic-wave-type apparatuses use highly directional electromagnetic waves in order to reduce effect of electromagnetic waves reflected on objects besides a surface of molten metal (line 16 in the upper and right-hand column on page 2 to line 3 in the lower and left-hand column on the same page of Patent Document 1 and paragraph [0037] of Patent Document 3). There also exists one provided with radio wave absorbing members on walls of the mold or the like for the purpose of anti-reflection (lines 2 to 9 in the lower and right-hand column on page 2 of Patent Document 2). Further, in conventional methods in which electromagnetic waves are used, electromagnetic waves of frequency which is 10 GHz or higher are used to downsize antennas for highly directional electromagnetic waves (lines 6 to 11 in the upper and right-hand column on page 3 of Patent Document 1 and paragraph [0019] of Patent Document 3).
However, electromagnetic wave of a higher frequency produces a large loss in signal through cable and is difficult to handle. Additionally, electromagnetic wave of a higher frequency requires an apparatus which is complicated in structure and expensive. Further, when highly directional electromagnetic wave is used, a change in level at a local portion of the surface of the molten metal affects measurement of a level of molten metal to an excessive degree, and therefore an apparatus for measuring a level of molten metal which is highly robust against a change in level at a local portion of the surface of the molten metal cannot be obtained. Thus, conventional apparatuses for measuring a level of molten metal are complicated in structure, expensive and poorly robust against a change in level at a local portion of the surface of the molten metal.